1. Field of the Invention
This invention relates to an electromagnetic coherent transient device and, more particularly, to a continuous processor that can simultaneously, asynchronously, and continuously process an input signal while it is continuously being programmed. The invention has application in, but is not limited to, the areas of memory, correlations/convolutions, true-time delay, pulse shaping, distortion compensation and spatial routing of optical signals.
2. Description of the Related Art
The present invention utilizes several unique properties of condensed phase spectral hole burning materials as versatile coherent transient processors. Absorption features of ions or molecules doped into condensed phase materials are spectrally broadened by two main classes of mechanisms. Homogeneous broadening is the fundamental broadening experienced by all ions or molecules independently, and arises from the quantum-mechanical relationship between the lineshape and the dephasing time of the excited ion. At cryogenic temperatures, such homogeneous linewidths have been measured, using the photon-echo technique, to be as narrow as 100 Hz or less, orders of magnitude sharper than most gas phase transitions (R. W. Equall, Y. Sun, R. L. Cone, R. M. Macfarlane, Ultraslow Optical Dephasing in Eu3+:Y2SiO5, Phys. Rev. Lett. 72, 2179 (1994)). Inhomogeneous broadening, such as depicted in FIG. 1A, arises from the overlap of the quasi-continuum of individual spectra of all of the ions or molecules in the condensed phase material, which have microscopically different environments and therefore slightly different transition frequencies. The extent of this envelope can be anywhere from hundreds of megaHertz (MHz) to the teraHertz (THz) range.
Spectral hole burning (W. E. Moemer, ed., Persistent Spectral Hole Burning: Science and Applications, (Springer-Verlag, Berlin 1988); R. M. Macfarlane and R. M. Shelby, xe2x80x9cCoherent Transient and Spectral Holebuming Spectroscopy of Rare Earth Ions in Solids,xe2x80x9d in Spectroscopy of Solids Containing Rare Earth Ions, A. A. Kaplyanskii and R. M. Macfarlane, eds. (North Holland, Amsterdam 1987)) uses a narrow-band laser to selectively excite only the small fraction of ions or molecules whose frequencies coincide with that of the laser. If some mechanism exists to remove those ions from the absorbing population, or to change their resonant frequencies, then the inhomogeneous absorption profile can be temporarily or permanently altered, leaving a xe2x80x9cspectral holexe2x80x9d or ion population shift at the frequency of the laser. In the cases of interest here, the homogeneous linewidth is many orders of magnitude smaller than the inhomogeneous linewidth. A very flexible advantage of hole burning, in contrast to isolated atomic transitions, is that the center frequency for the hole may be chosen anywhere within the wide inhomogeneous band of absorbing frequencies. Furthermore, multiple holes or ion population variations may be placed within one inhomogeneously broadened absorption band to generate an absorption spectrum. In addition, by varying the intensity of the burning spatially, as with an interference pattern among two or more laser beams, a spatial-spectral grating can be produced.
Mechanisms exist to provide permanent change to preserve the hole; the most common being (a) excitation-induced changes in the lattice near the optically active ion or molecule, (b) photoionization of that ion or molecule itself, and (c) photochemical reactions. Possibilities exist to use photon-gated processes where a second, possibly broadband, light source is required to make the hole persistent.
An electromagnetic wave coherent transient device is one with a broadband spectral grating that extends over several homogeneous profiles, and part or all of the available inhomogeneous broadening absorption profile. An optical coherent transient (OCT) device is one with a broadband spectral grating in the optical range that extends over several homogeneous profiles, and part or all of the available inhomogeneous broadening absorption profile. All the components of an optical spectral grating are typically programmed simultaneously by recording the spectral-spatial interference of two or more optical pulses separated in time and/or space. This spectral grating has the ability to generate an optical output signal that depends on an optical input waveform (referred to as a processing waveform) impinging on that grating, now referred to as a device. Optical coherent transient (OCT) devices have been disclosed such as an optical memory (for example, T. W. Mossberg, xe2x80x9cTime-Domain Frequency-Selective Optical Data Storage,xe2x80x9d Opt. Lett. 7, 77 (1982)), a swept carrier optical memory (for example, T. W. Mossberg, xe2x80x9cSwept-Carrier Time-Domain Optical Memory,xe2x80x9d Opt. Lett. 17, 535 (1992)), an optical signal cross-correlator (for example, W. R. Babbitt and J. A. Bell, xe2x80x9cCoherent Transient Continuous Optical Processor,xe2x80x9d Appl. Opt. 33, 1538 (1994)), an optical true-time delay regenerator (see for example, K. D. Merkel and W. R. Babbitt, xe2x80x9cOptical Coherent Transient True-time Delay Regenerator,xe2x80x9d Opt. Lett. 21, 1102 (1996)) and optical spatial router (for example, W. R. Babbit and T. W. Mossberg, xe2x80x9cSpatial Routing of Optical Beams Through Time-domain Spatial-spectral Filtering,xe2x80x9d Opt. Lett. 20, 910 (1995)), among others. While each device has different aspects in its programming and processing stages, all are implementations of a generalized OCT processor. The term processor here indicates the most generalized conceptualization of such a device.
OCT devices can only process data as long as the programmed spatial-spectral grating survives. When the programming stage is a single shot event, writing a strong spectral grating in a non-persistent hole-burning material, the processing stage can only occur while the excited absorbers have not decayed back to their ground states. After the grating decays away fully, the programming stage can be repeated, but this leads to dead time for the processor, which is several times the excited state lifetime T1. An alternative implementation is to utilize persistent spectral holes. But for single photon persistent holes (e.g., hyperfine storage), the processing stage is partially destructive to the stored gratings, (for example, M. Zhu, W. R. Babbitt and C. M. Jefferson, xe2x80x9cContinuous Coherent Transient Optical Processing in a Solid,xe2x80x9d Opt. Lett. 20, 2514 (1995)). For two-photon persistent holes, i.e., gated storage, (for example, W. E. Moemer, Editor. Persistent Spectral Holeburning: Science and Applications, Topics in Current Physics, Vol. 44, Springer-Verlag, 1988), the processing stage is non-destructive and can be continuous. However, low writing and gating efficiencies of available materials make this currently impractical. Each of the above techniques requires strong programming pulses and weak processing pulses, thereby putting stringent specifications on the laser source and optical modulators. These constraints could be lessened if the grating were accumulated by repeating the programming process. However, the temporal distinction between the programming and processing stages still remains for all the above techniques, along with the constraints on efficiency, materials and devices.
Previously, a continuous OCT processor has been proposed and demonstrated whereby the programming stage and the processing stage are two temporally distinct steps. Furthermore, previous continuous OCT processors required two spatially distinct programming beams to write a spatial-spectral population grating and a third beam to read. However, in the previous OCT processor, the first and third beams are typically specified to be the same, which makes the emitted output signal spatially distinct from the subsequently applied continuous processing data stream. Consequently, the programming and processing stages cannot be overlapped in time.
Previously, storage of the grating required a persistence of the spectral holes at least as long as the processing stage step. Herein spectral holes that naturally persist much longer than the processing stage step without further programming are called persistent spectral holes, and the remaining holes are termed non-persistent spectral holes.
Previously, the efficiency of the system for long term storage was greatly limited by the material""s ability to efficiently transfer electron populations to persistent states.
Previously, reprogramming required either a means by which to erase or to decay away the stored grating, or, to change the spatial location where a grating is stored so as to address new atoms.
A previously known technique for accumulated gratings for photon echoes, is used to see extremely weak photon echo signals by repeated application of two brief reference pulses (for example, W. H. Hesselink and D. A. Wiersma, xe2x80x9cPhoton Echoes from an Accumulated Grating: Theory of Generation and Detection,xe2x80x9d J. Chem. Phys., 75, 4192 (1981)). Accumulated gratings have never been previously considered for processing, nor has the advantage of high efficiency gratings for optical processing been noted in any publication or in the art known to the inventors.
There is a need for a device in which the programming and processing can occur continuously, simultaneously and asynchronously.
It is an objective of the present invention to provide a device in which the programming of a spatial-spectral grating and processing using that grating can occur continuously, simultaneously and asynchronously. It is a feature of one aspect of the present invention to provide a method for achieving continuous processing in materials that do not exhibit long term persistent hole burning. A grating is accumulated and maintained regardless of the material""s ability to transfer ion populations to persistent states. In the present invention, the material adapts to new programming pulses in a continuous manner within the grating lifetime, either the excited state lifetime T1, or the intermediate state lifetime TB in the case of a three-level system, if TB greater than T1.
It is a feature of another aspect of the present invention to accumulate and repeatedly refresh the stored grating by repeatedly applying the programming sequence by repeated application of two or more spatially distinct programming pulses to a non-persistent hole-burning material.
It is a feature of another aspect of the present invention to provide an input beam geometry that isolates the emitted signal from all the input waveforms so that the optical signal processor can process simultaneously, asynchronously, and continuously a processing input signal while it is being continuously programmed.
It is a feature of one aspect of the present invention to provide an input beam geometry that has the benefit that non-linearities introduced by the multiple programming stages do not lead to harmonics of the delay in the output signal direction in, at least, the case of true time delay regenerators.
It is a feature of one embodiment of the present invention to provide two spatially distinct programming beams to write a spatial-spectral population grating and to provide a third spatially distinct processing beam, whereby the emitted output signal is spatially distinct from the subsequently applied continuous input data stream on the processing beam, and the programming beams, and thus can temporally overlap them.
It is a feature of another embodiment of the present invention to provide three (or more) spatially distinct programming beams to write a spatial-spectral population grating and to provide a fourth (or higher) spatially distinct processing beam, whereby the emitted output signal is spatially distinct from the subsequently applied continuous input data stream on the processing beam, and the programming beams, and thus can temporally overlap them.
It is a feature of another embodiment of the present invention to provide for angular demultiplexing and multiplexing of gratings stored in a particular spatial location by varying the beam directions of one or more of the programming pulses, respectively, while still maintaining the phase matching conditions of the input pulse geometry.
It is a feature of another embodiment of the present invention to provide a system that can fully process a temporally structured waveform (TSW) modulated in amplitude and phase at projected data rates typically greater than 10 GHz and with time-bandwidth products typically greater than 1000.
It is a feature of the present invention to provide efficient and continuous processing in an absorptive material system that has an inhomogeneously broadened transition (IBT) spectrum between ion states. The system can be, but is not limited to, a two-level transition, three-level (multi-level) with one (or more) intermediate states between the upper and lower levels of the IBT, or multi-level with transitions outside of the IBT, as applicable for optical pumping.
It is a feature of an aspect of the present invention to provide for reprogramming of the processor simply by changing the programming pulses. This feature also allows the processor to adapt to frequency drift of the system""s laser source and can be used in adaptive learning of unknown patterns.
In brief, a novel method and system for programming and processing coherent transient devices is described herein. The method and system utilizes distinct processing and programming beams, making it possible to simultaneously program a grating and process a waveform against it, continuously and asynchronously. The method and system has utility in the processing of optical waveforms for optical or electronic means. New delays, patterns or other spectral gratings can be programmed into the material to a steady state by accumulation, with a programming time on the order of the grating lifetime. New delays, patterns or other spectral gratings can be programmed to their steady state value in a single shot, quickly, and then maintained at that steady state value with continuously applied programming waveforms. Reprogramming of new delays, patterns or other spectral gratings can be achieved with reprogramming time roughly equal to the grating lifetime. Quicker methods for reprogramming a spectral grating can be utilized as well. These programming methods alleviate the need for photon gating in several types of OCT devices, specifically optical memory (i.e. optical DRAM), temporally structured waveform cross-correlators, and true-time delay regenerators. For both pattern storage, as used in a memory and for correlation processing, and for a true-time delay regenerator, simulations show the efficiency to be on the order of that for a perfect photon-gated system. Existing materials offer multi-gigahertz, efficient, real-time processing with large time bandwidth products ( greater than 1000).
The foregoing needs and objects, and other needs and objects that will become apparent from the following description, are achieved by the present invention, which comprises, in one aspect, an electromagnetic wave coherent transient device having a medium with an inhomogeneously broadened transition absorption spectrum and with a relaxation time during which modifications to the inhomogeneously broadened transition absorption spectrum substantially decay away. A plurality of programming paths impinge on the medium from a plurality of corresponding programming path directions. A processing path impingies on the medium from a processing direction. An output path emanates from the medium in an output direction. The output direction, the processing direction and each direction of the plurality of corresponding programming path directions are phase matched so the output direction is different from the processing direction and different from each direction of the plurality of corresponding programming path directions.
In another aspect of the invention, an electromagnetic wave coherent transient device also includes a medium having an inhomogeneously broadened transition absorption spectrum and a relaxation time during which modifications to the inhomogeneously broadened transition absorption spectrum substantially decay away. In this embodiment there is a programming source of a plurality of modulated programming pulses to form a modified transition absorption spectrum in the medium, and a processing signal source. The processing signal source is configured to send a processing signal onto a certain location in the medium during a processing time interval. The programming source is configured to send the plurality of modulated programming pulses onto a location that at least partially overlaps the certain location during a programming signal time that overlaps in time the processing time interval.
Another aspect of the invention is a method of using a coherent electromagnetic wave transient device. The device has a medium with an inhomogeneously broadened transition absorption spectrum and a relaxation time during which modifications to the inhomogeneously broadened transition absorption spectrum substantially decay away. The device also has a plurality of programming paths impinging on the medium from a plurality of corresponding programming path directions, a processing path impinging on the medium from a processing direction, and an output path emanating from the medium in an output direction. The method includes sending a plurality of programming pulses along the plurality of programming paths during a programming signal duration to form a modified transition absorption spectrum in the medium. A processing signal of arbitrary duration that overlaps in time at least one programming pulse of the plurality of programming pulses is sent along the processing path. An output signal of arbitrary duration is received along the output path in response to the processing signal.
In another aspect of the invention, an optical coherent transient device includes one or more lasers emitting optical radiation at a carrier frequency. The optical radiation is modulated to form a processing waveform on a processing path and a plurality of programming pulses on a plurality of corresponding programming paths. A medium has an inhomogeneously broadened transition absorption spectrum and a relaxation time during which modifications to the inhomogeneously broadened transition absorption spectrum substantially decay away. The processing signal impinges onto a certain location in the medium during a processing time interval. A programming signal impinges onto a location that at least partially overlaps the certain location during a programming signal time that overlaps in time the processing time interval. The programming signals form a modified transition absorption spectrum.
In another aspect of the invention, an optical coherent transient device includes one or more lasers emitting electromagnetic radiation at a carrier frequency. A processing waveform is formed on a processing path. A plurality of programming pulses are formed on a plurality of corresponding programming paths. A medium has an inhomogeneously broadened transition absorption spectrum and a relaxation time during which modifications to the inhomogeneously broadened transition absorption spectrum substantially decay away. The plurality of corresponding programming paths impinge on the medium from a plurality of corresponding programming path directions. The processing optical path impinges on the medium from a processing direction. An output optical path emanates from the medium in an output direction. The output direction, the processing direction and each direction of the plurality of corresponding programming path directions are phase matched so the output direction is different from the processing direction and different from each direction of the plurality of corresponding programming path directions.